The subject application relates to document scanning, and more particularly to scanner and printer calibration, and more particularly to mitigating metamerism in color printing and/or scanning.
Previously, scanners used only RGB or Clear: clear for grayscale scanning; RGB for color. By restricting the color set in CMYK space, RGB to L*a*b* conversions can be obtained, however some colors remain with errors greater than 2 delta-E (DE) units. One reason for poor conversions is fluorescence in the paper, causing the color to depend on the amount of paper covered, in ways that are not directly describable in a three-primary space.
Some scanners are equipped with four channels: Red, Green, Blue and Clear (RGBW). In principle, when scanning a page with only CMY used to control color, RGB would be sufficient to capture desired information, so long as the paper and toner set do not change. This principle has been exploited in scanner-based printer calibration techniques, since printer calibrations generally only use K in the absence of CMY and various combinations of CMY (without K). By characterizing the scanner separately for K and for different regions of CMY space, scanner characterizations have been obtained at 0.51 mean DE 2000, with a 95th percentile of 1.27. However, the darkest patches tend to be beyond the 95th percentile, with values ranging from 2.4 to 6.18. Furthermore, to obtain such values requires holding the toner set and paper fixed, and restricting the colors to those normally used in printer calibration. One reason that RGB is insufficient to completely capture CMY variation is fluorescence: as more of the page is covered, less of the detected light results from fluorescence, and the hue of the page can shift. An indication of this is the fact that the hue changes along a step wedge of any single separation.
The clear channel uses the same sensors and illuminant as the RGB channels: there is only limited information available “in the cracks.” Suppose, for example, that the RGB filters were perfect block dyes, matched so that each one transmitted perfectly over a distinct sub-range of the visible, and exactly one transmitted at each wavelength in the range. In that case the RGB signals could be combined (using a weighted average) to predict the clear signal. For two reasons, the prediction is imperfect. First, the filters are not perfect block dyes. Some information is lost when sampling in RGB space, due to metamerism, and a clear channel, even though it is metameric itself, can, when combined with the RGB channels, restore some of that information. Second, when one of the three RGB channels is weak, the sensor has a low signal to noise ratio. Accordingly, there is an unmet need for systems and/or methods that facilitate overcoming the aforementioned deficiencies.